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Here are a couple of great Preview Handlers for PDF files.
Foxit PDF Preview Handler as well as Adobe PDF Preview Handler.
Another one is in this MSDN article. The code download for this article includes a framework that makes it a snap to implement your own preview handlers, and it provides several sample previewers (including previewers for PDF, XML, ZIP, MSI, BIN, CSV, XPS, and XAML files).
If I had more time I would write one for DWG and DWF files. With the above code samples it's a quite easy task.
